As is generally well-known, many electrical or electronic packages having semiconductor devices generate high heat dissipation which may be destructive to the semiconductor device electrical properties. In order to carry away the heat generated by semiconductor devices, the electrical or electronic package includes a heat sink member which is often desired to be encapsulated within a plastic material. It has been found that the plastic encapsulation of the heat sink member substantially reduces the cost of the final assembly.
For encapsulating in plastic by an injection insert-molding process, an upper mold plate and a lower mold plate forming a cavity therebetween is used. The heat sink insert is placed in the mold and then the two plates of the mold are closed. A molten plastic is then forced into the cavity in a well-known manner and hardened about a portion of the heat sink to form a finished plastic electronic package. Thereafter, the two plates of the mold are opened and the finished package is ejected.
The problem with this prior art technique is that during the injection stage of the process, an interference fit between the heat sink insert and the mold cavity is required. The upper mold plate and the lower mold plate are machined to very tight tolerances. However, it is more difficult to maintain the heat sink inserts to the same tight tolerances. The heat sink insert, formed with an undercut as required for mold encapsulating purposes, is not capable of being die cast within the critical molding tolerances.
If the heat sink insert is undersized, this will cause the mold cavity to have an excessive clearance so that a thin layer of the encapsulating plastic material will squirt out between the cavity and the insert in what is commonly referred to as a flashing condition. This flashing condition must then be removed by an expensive secondary machining operation, which not only increases the manufacturing costs, but may cause damage to the finished package. On the other hand, if the heat sink insert is oversized, this will cause loading into the cavity to be difficult, creating a misalignment condition leading to mold damage as well as damage to the finished package. Thus, the finished package will not be usable and must be scrapped.
In the alternative, the heat sink insert could be machined to a tight tolerance so as to be acceptable. However, this operation would also increase the manufacturing costs.
It would, therefore, be desirable to provide an improved method of encapsulating a die cast insert so as to eliminate flashing being formed on the surfaces of the insert. It would also be expedient that the method of molding compensates for dimensional variations of the insert being encapsulated.